If a single, inorganically grown apple only had a single drop of pesticide on it — as it is by itself this is not ideal, but not catastrophically dangerous — it would literally take 500 of these pesticide-laced apples to cause a frightening reaction.
However, in the process of extractions (like those used in medicinal cannabis extraction), 1,000 apples are essentially smashed together and condensed down and put into a shot glass and made very easy to consume all at once. Now you have a circumstance where you can be exposed to a massive amount of a dangerous substance.
This is essentially the concern when it comes to patients using medical cannabis products not sourced from farms with organic growing methods.
As more people move away from the traditional consumption methods of cannabis, the extraction and refinement of these products warrant responsibility back to the growers.